1. Field of the Invention
The present invention relates to an amplifier circuit, particularly to an amplifier circuit including a problem such that a distortion occurs in spread spectrum communication.
2. Description of the Related Art
There will now be described a linear amplifier as one example of a conventional amplifier circuit. The linear amplifier inputs a low level signal, linearly amplifies the signal, and subsequently outputs a desired signal. In general, the linear amplifier comprises a differential amplifier. When the differential amplifier is supplied with a voltage signal and outputs a current signal, a load resistor converts the current signal to a voltage. A current gain ΔI1 of the differential amplifier in voltage-to-current conversion is represented by the following equation using Vin as an input signal amplitude.                               Δ          ⁢                                           ⁢                      I            1                          =                  A          ·                                    tan              ⁢              h                        (                                          V                                  i                  ⁢                                                                           ⁢                  n                                                            2                ⁢                                  V                  T                                                      )                                              (        1        )            Here A denotes a current value of a constant current source of a differential pair of transistors, and VT denotes a thermal voltage. When tanh␣x is approximated, tanh␣x≈x-x3/3 can be represented. Therefore, the equation (1) can be represented as follows.                               Δ          ⁢                                           ⁢                      I            1                          =                  A          ⁡                      (                                                            V                                      i                    ⁢                                                                                   ⁢                    n                                                                    2                  ⁢                                      V                    T                                                              -                                                1                  3                                ⁢                                                      (                                                                  V                                                                                                                                   ⁢                                                      i                            ⁢                                                                                                                   ⁢                            n                                                                                                                      2                        ⁢                                                  V                          T                                                                                      )                                    3                                                      )                                              (        2        )            Here a second term indicates a distortion component. Particularly a third-order intermodulation distortion (IM3) poses a largest problem as an adjacent channel leakage power of the signal in a spread spectrum radio system. The distortion is generated by presence of the term. Since VT is 26 mV at room temperature, and when IM3 is lowered to −60 dBc or less as a condition for distortion reduction, Vin needs to be 2.8 mV or less. Therefore, the aforementioned linear amplifier has a reduced distortion only when the input signal amplitude is very small. The linear amplifier cannot be utilized in an amplifier in which a signal to be handled is large in a range of 10 mVpp to 1 Vpp, particularly in a power amplifier.
The third-order intermodulation distortion as a main factor of distortion of the amplifier increases when an output power is increased. Therefore, in order to amplify the signal at the reduced distortion, a method of reducing the output power per one stage of the amplifier and obtaining a gain by a multi-stage structure of an amplifier is used. However, this poses problems such as an increase of power consumption, increase of a mounting area by an increase of the number of chips, and cost increase.
It is an object of the present invention to provide an amplifier circuit in which the third-order intermodulation distortion is inhibited without suppressing the output power.